An active matrix substrate for use in a liquid crystal display device or the like includes a switching element, such as a thin film transistor (hereinafter, “TFT”), in each pixel. As such a switching element, a TFT which includes an amorphous silicon film as the active layer (hereinafter, “amorphous silicon TFT”) and a TFT which includes a polycrystalline silicon film as the active layer (hereinafter, “polycrystalline silicon TFT”) have been widely used in conventional devices.
Active matrix substrates usually have a display region which includes a plurality of pixels and a region exclusive of the display region (peripheral region). Each pixel of the display region includes a source wire extending in a column direction of the pixels, a gate wire extending in a row direction of the pixels, a pixel electrode, and a TFT. In the peripheral region, a plurality of terminal portions are provided for connecting the gate wires or source wires to external wires. For example, the gate wires extend from the display region to the peripheral region and are connected with a gate driver via the terminal portions (gate terminals). Meanwhile, the source wires are, for example, electrically coupled with gate connecting wires formed out of a same film as the gate wires. This connecting portion is referred to as “source-gate connecting portion”. The gate connecting wires are connected with a source driver via the terminal portions (source terminals) in the peripheral region. Wires, such as gate wires, source wires, and gate connecting wires, are metal wires, for example. In this specification, structures for connecting wires with each other, such as gate terminal portions, source terminal portions, and source-gate connecting portions, are generically referred to as “wire connecting portions”.
In recent years, using an oxide semiconductor as a material of the active layer of TFTs, instead of amorphous silicon and polycrystalline silicon, has been proposed. Such TFTs are referred to as “oxide semiconductor TFTs”. The oxide semiconductor has higher mobility than the amorphous silicon. Therefore, oxide semiconductor TFTs are capable of higher speed operation than amorphous silicon TFTs. Further, oxide semiconductor films can be formed through a simpler and more convenient process than polycrystalline silicon films and are therefore applicable to devices which require large surfaces.
As an oxide semiconductor TFT, a structure which has, for example, a bottom gate configuration and in which a protection layer (etch stop layer) is arranged so as to cover a channel region of an oxide semiconductor layer has been proposed. Such a structure is referred to as “channel protection type (or etch stop type)”. In a manufacturing process of an etch stop type TFT, source/drain electrodes are formed after a protection layer is formed on an oxide semiconductor layer. Thus, in etching for formation of the source/drain electrodes (source/drain separation), the protection layer functions as an etch stop, so that damage to the channel region from the etching can be reduced.
For an active matrix substrate which includes etch stop type oxide semiconductor TFTs, the step of forming a protection layer is added. Various processes for manufacture of such an active matrix substrate with a reduced number of photomasks have been studied (e.g., Patent Document 1).